Ebnerite and epiebnerite: NH 4 ZnPO 4 dimorphs with zeolite-type frameworks from the Rowley mine, Arizona, USA

• Published in: Mineralogical magazine 2024-06, Vol.88 (3), p.312-318
• Main Authors: Kampf, Anthony R., Gu, Xiangping, Yang, Hexiong, Ma, Chi, Marty, Joe
• Format: Article
• Language: English
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/mgm.2024.15

Ebnerite and epiebnerite, both with the ideal formula NH 4 ZnPO 4 , are new mineral species from the Rowley mine, Maricopa County, Arizona, USA. They occur in an unusual bat-guano-related, post-mining assemblage of phases. Epiebnerite grows epitactically on ebnerite and replaces it. Ebnerite and epiebnerite are found in intimate association with alunite, halite, mimetite, newberyite, sampleite, struvite and wulfenite on hematite-rich quartz–baryte matrix. Crystals of ebnerite are colourless narrow prisms up to ~0.3 mm in length. The streak is white, lustre is vitreous, Mohs hardness is ~2, tenacity is brittle and fracture is splintery. The density is 2.78(2) g⋅cm –3 . Ebnerite is optically uniaxial (–) with ω = 1.585(2) and ɛ = 1.575(2). Epiebnerite occurs as colourless prisms or blades, up to about 10 × 3 × 2 μm, in parallel growth forming ribs with serrated edges epitactic on ebnerite prisms. The streak is white, lustre is vitreous, Mohs hardness is probably ~2, tenacity is brittle. The calculated density is 2.851 g⋅cm –3 . Epiebnerite is optically biaxial with all indices of refraction near 1.580. Electron microprobe analysis gave the empirical formula [(NH 4 ) 0.89 K 0.06 ] Σ0.95 (Zn 0.96 Cu 0.07 ) Σ1.03 [(P 0.97 Si 0.03 ) Σ1.00 O 4 ] for ebnerite and [(NH 4 ) 0.67 K 0.28 ] Σ0.95 (Zn 0.99 Cu 0.02 ) Σ1.02 (P 1.00 O 4 ) for epiebnerite. Ebnerite is hexagonal, P 6 3 , with a = 10.67051(16), c = 8.7140(2) Å, V = 859.25(3) Å 3 and Z = 8. Epiebnerite is monoclinic, P 2 1 , with a = 8.796(16), b = 5.457(16), c = 8.960(16) Å, β = 90.34(6)°, V = 430.1(17) Å 3 and Z = 4. The structures of ebnerite ( R 1 = 0.0372 for 1168 I o > 2σ I reflections) and epiebnerite (known from synthetic monoclinic NH 4 ZnPO 4 ) are zeolite-like frameworks based upon corner-sharing linkages between alternating ZnO 4 and PO 4 tetrahedra with channels in the frameworks hosting the NH 4 groups. The two structures are topologically distinct. Ebnerite belongs to the family of ‘stuffed derivatives’ of tridymite, whereas epiebnerite possesses an ABW-type zeolite structure.